The implementation of ball grid arrays has become more extensive. Ball grid arrays have been used to provide high input/output on printed circuit boards with less space while providing a rugged package that eliminates handling problems associated with fine-pitch devices.
PCB assembly processes, that employ ball grid arrays, typically start with a screen print operation with stencils to place solder paste on pads. The board typically passes to a pick and place machine which places components on paste at the bottom side of the board. After the board goes through a reflow oven, it then goes through a top side paste operation and then a top side pick and place operation. The board is placed in a reflow oven after each pick and place operation and subjected to a soak period in which the board is ramped up to a temperature (e.g. 126.degree. C.). In a subsequent zone the temperature is brought up to the reflow temperature to form the solder joints. Next a cool down process is provided, particularly a slow cool down to provide good metallurgical properties to the board. There are many more operations which are typically provided, including testing. However, the operations noted are the basic operations of printed circuit assembly.
Problems can result with the final product including various different defects. The pins can be rotated, there can be poor joints and elements can be skewed. In such situations, a rework process may be followed to take the board off the ball grid array. Ball grid array that are removed from the board are typically not reused.
During rework, temperature profiling allows the ramp up of the ball grid array case and joints to temperatures of 100.degree. C. via a thermal couple based event. After soaking at this level form one to three minutes, temperatures reach 120.degree. to 135.degree. C. at which the upper nozzle temperature and flow rate are set so the joints are a brought to 175.degree. C. sea before reflow. This makes it easier to quickly ramp to the temperature without exceeding maximum allowable case temperatures or exceeding the 90 sec. limit above liquidus temperature. Profiling is also used to achieve the joint temperatures of 208.degree. to 218.degree. C., exceeding the case temperature, which is usually 230.degree. C. to 235.degree. C. Once the profile is optimized, several electrically rejected grid array should be installed and removed to verify the integrity of the process.
The publication "SUCCESSFUL GUIDELINES FOR BGA REWORK" describes rework operations and how they should replicate the original assembly process as closely as possible. A fully populated circuit board together with a minimum of five electrically rejected BGAs are provided for profiling. Since most ball grid arrays will be reworked on a populated board, a populated board is preferred over a bare board. Thermal couples (thermocouples) with wire diameter of 0.005 in. are placed into 0.018 in. drilled holes on the BGAs. The junction of the BGA is just above the pad surface, which is the joint created during the rework process. Junctions (Thermal couple probes) should be less than 0.006 in. above the board to insure the rise through the paste is prevented when micro-stenciling on to the site. The thermal couple is firmly attached to the board using tape. The thermal couple junction is often re-inspected to using magnification to position and center in the thermal couple on pad. If not centered, it may be necessary to remove the thermal couple and reapply the tape. This process can be quite tedious. However, it is absolutely essential to provide a good profile as part of a good rework process.
As the rework process represents additional thermal cycles a proper profile should be created ideally using the same profile board as used in the reflow oven. Sensing critical location temperatures should be provided to insure the vendors maximum allowable case temperatures are not exceeded during the rework process. However, the sensing involves the use of thermal couples strategically positioned wherein precise measurements are required. This can be problematic with regard to physical and system difficulties in obtaining a good profile.